This invention relates to frequency modulation (FM) detectors and, more particularly, to FM detectors for radio receivers.
There are a significant number of prior FM detector arrangements known in the art. Indeed, binary differential detection of FM signals has been recognized as a very desirable technique for recovering FM data signals in radio receivers. Heretofore, such differential detectors have employed super heterodyne techniques for obtaining the necessary intermediate frequency. A serious limitation of super heterodyne detectors is that they can not be implemented totally in integrated from. This is because required image-reject and channel select filters would necessarily be implemented off the integrated circuit chip including the other receiver circuitry.
One attempt at overcoming the limitations of the super heterodyne detector approach is described in U.S. Pat. No. 4,750,214 issued Jun. 7, 1988. In the disclosed arrangement FM signals are demodulated by first converting the FM signal using an analog-to-digital (A/D) converter. Then, a quadrature mixing arrangement is used to obtain the base band in-phase and quadrature phase components of the received data signal. Then, the in-phase and quadrature phase signal components are delayed by a sample interval and cross-products of both the delayed in-phase component and quadrature phase signal component, and the delayed quadrature phase signal component and the in-phase signal component are obtained. The algebraic difference of these signal products is obtained which yields the desired data output signal y.
There a number of problems with the prior differential FM detector arrangements employing the super heterodyne techniques when attempting to apply them to low intermediate frequency (IF) radio receivers. Specifically, generation of a phase shift of xcfx80/2 over the whole signal band is problematic at low IF frequencies. Because of the lower IF frequency, double frequency terms that result in differential FM detectors cannot adequately be removed and, consequently, cause a degradation of the detector performance. Additionally, if a received signal is merely limited before it is supplied to the detector, the resulting harmonics are now located closely and also cause interference. Furthermore, implementing analog delay units for use at a relatively low frequency is also problematic.
Moreover, in GFSK (Gaussian Frequency Shift Keyed) data detectors spreading of a data symbol in time over less than two (2) symbol periods causes intersymbol interference (ISI). This interference has an undesirable negative effect on the BER (bit error rate) performance of the detector.
These and other problems and limitations of prior known arrangements for differential FM detection of signals are realized in a differential FM detector that uses in-phase and quadrature phase signal components of a received signal, wherein the in-phase and quadrature phase signal components are at a low intermediate frequency (IF). Both the in-phase and quadrature phase signal components are amplitude limited, sampled at a prescribed sampling rate and filtered in a prescribed manner. Delayed versions of the filtered in-phase and quadrature phase signal components are generated and, then, signal products are generated of the delayed in-phase signal component and quadrature phase signal component, and the delayed quadrature phase signal component and in-phase signal component. The algebraic difference of the generated signal products is obtained to yield the desired data signal, e.g., symbols.
Specifically, a FIR (finite impulse response) filter is employed to filter the limited and sampled versions of the in-phase and quadrature phase signal components to alleviate the interference caused by the limiter.
In an embodiment of the invention, the detector is a differential Continuous Phase Frequency Shift Keyed (CPFSK) FM detector.
In a GFSK differential FM detector, employing a post detection compensation arrangement significantly reduces distortion caused by ISI. Specifically, the absolute value of the detected data signal is obtained and compared to a prescribed threshold value. If the threshold is exceeded no compensation is required. However, if the threshold is not exceeded, it is assumed that prescribed data symbol transitions have occurred and that the currently received data symbol is the inverse of the last preceding detected data signal. Consequently, the current data symbol is replaced by the inverse of the preceding data symbol.